Transferrin receptor functions as a signal-transduction molecule for its own recycling via increases in the internal Ca2+ concentration.
Transferrin binding to its receptor modulates transferrin receptor (Tf-R) recycling rates in several cells [Klausner, R. D., Van Renswoude, J., Ashwell, G., Kempf, C., Schechter, A., Dean, A. & Bridges, K. R. (1983a) J. Biol. Chem. 258, 4715-4724; Gironès, N. & Davis, R. J. (1989) Biochem. J. 264, 35-46; Sainte-Marie, J., Vidal, M., Bette-Bobillo, P., Philippot, J. R. & Bienvenüe, A. (1991) Eur. J. Biochem. 201, 295-302]. To delineate the mechanism of this regulation, we hypothesized that the binding of the ligand to its receptor could lead to activation of several second-messenger pathways, which may redundantly stimulate recycling of the receptor. The effects of different regulators of Ca2+ flux or concentrations were investigated on the Tf-R-recycling pathway; these studies were carried out in two cell types. Perhexiline, a calcium antagonist, slowed receptor recycling in comparison with the control by more than 80% in L2C cells and by 60% in Jurkat cells (B and T lymphoblasts, respectively) but did not affect their internalization rate. Perhexiline thus trapped considerable amounts of Tf-R in the internal compartment. Ca2+ chelators, such as EGTA or 1,2-bis(2-aminophenoxy)ethane-N,N,N,N'-tetraacetic acid, and a Ca2+-channel inhibitor (Ni2+) decreased drastically the recycling rate of Tf-R. Tf-R recycling was shown to be slowed by a calmodulin antagonist. Conversely, artificial elevation of free internal Ca2+ in L2C cells, using lectin, accelerated the recycling rate. These results suggest that the intracellular Ca2+ concentration plays an important role in the outward flow of transferrin receptors. Consequently, we examined the role of transferrin in internal free Ca2+ regulation. The addition of transferrin or anti-(Tf-R) Ig specifically elicited a rise in [Ca2+], as demonstrated by inefficacy of apotransferrin or irrelevant antibodies. These results suggest that Ca2+ is a regulator of Tf-R recycling and that Tf-R seems to function as a signal-transduction molecule (perhaps in conjunction with other membrane proteins) rather than merely as an endocytic receptor.
Sainte-Marie
J
J
UMR 5539 CNRS, Université Montpellier II, France. stemarie@univ-montp2.fr
Lafont
V
V
Pécheur
E I
EI
Favero
J
J
Philippot
J R
JR
Bienvenüe
A
A
D000906:Antibodies / Q000276:immunology / Q000494:pharmacology; D001059:Apoproteins / Q000494:pharmacology; D002118:Calcium / Q000032:analysis / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002147:Calmodulin / Q000037:antagonists & inhibitors; D002460:Cell Line; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D006801:Humans; D007457:Iodine Radioisotopes / Q000378:metabolism; D009532:Nickel / Q000494:pharmacology; D010480:Perhexiline / Q000494:pharmacology; D010835:Phytohemagglutinins / Q000494:pharmacology; D011485:Protein Binding; D011990:Receptors, Transferrin / Q000276:immunology / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology*; D013936:Thymidine / Q000378:metabolism; D014168:Transferrin / Q000378:metabolism / Q000494:pharmacology; D014268:Trifluoperazine / Q000494:pharmacology
D000906:Antibodies; D001059:Apoproteins; D002121:Calcium Channel Blockers; D002147:Calmodulin; D002614:Chelating Agents; D007457:Iodine Radioisotopes; D010835:Phytohemagglutinins; D011990:Receptors, Transferrin; D014168:Transferrin; C028284:apotransferrin; D014268:Trifluoperazine; D004533:Egtazic Acid; D009532:Nickel; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D010480:Perhexiline; D002118:Calcium; D013936:Thymidine